The present invention relates to a code division multiple access mobile communication method and a system thereof, and especially, to a technology of handover.
In a mobile communication system, various kinds of multiple access techniques are adopted. A code division multiple access (referred to as CDMA hereinafter) mobile communication system that is one of them uses the same carrier frequency in all areas, and however, in order to discriminate areas covered by respective radio base stations from each other, a peculiar spreading code is allocated to every radio base station.
In this CDMA mobile communication technique, first, a mobile terminal device searches cell discriminating signals being transmitted from the radio base stations, and selects a radio base station having the strongest electric field strength of a radio wave.
Next, in case that conversation is conducted by the mobile terminal device, transmission and reception of a control channel signal such as a dial signal and a termination response signal are conducted by a control channel of the selected radio base station, and after it is conformed at an exchange control station that the above-described mobile terminal device can provide service, voice channels for the conversation are allocated.
By the way, even in case that, after the conversation is started by the mobile terminal device, a user (mobile terminal) moves from an area covered by the initial radio base station to an area covered by other radio base station, a handover method is adopted for preventing the conversation from being broken.
With regard to a technology in relation to this handover method, several methods have been proposed.
For example, in a mobile communication technique described in JP-P1997-307943A, in order to reduce a time period and increase a success rate of handover, a method has been proposed, in which a radio base station during conversation and an adjacent radio base station are caused to be a plurality of conference conversations in advance, and a handover reservation is made.
Also, with regard to a hand-off method to a different communication technique cellular system base station in a CDMA cellular system and a cellular system described in JP-P1999-355831A, in a mobile communication system in which a cellular system having a technique different from the CDMA cellular system is intermixed, and service is provided, when the reception electric field strength of a pilot signal transmitted from a radio base station exceeds a pre-set level, a mobile device being communicating with a CDMA cellular system radio base station measures this and makes a report. The reception electric field strength of this pilot signal is compared with the reception electric field strength of a signal transmitted from an adjacent radio base station of the other CDMA technique cellular system under a relationship condition. And, a proposal has been made, wherein, in case that this comparison result coincides with a preset condition most suitable for applying hand-off to a cellular system radio base station of a different technique, hand-off processing to the different technique cellular system radio base station is executed.
Also, in a CDMA mobile communication technique described in JP-P1999-262043A, a proposal has been made, wherein an exchange control station receives a dial signal or a termination response signal from a mobile device by way of a radio base station, and when it is determined based on adjacent cell electric field strength information that the mobile device exists in a boundary region of an adjacent cell, it transmits use control channel information of the radio base station to a radio base station of the adjacent cell, and the radio base station of the adjacent cell starts up a control channel in accordance with the control channel information, and the exchange control station transmits down-control channel signals to the mobile device and a radio base station adjacent to the radio base station during the transmission, respectively, and at the radio base station and the radio base station of the adjacent cell, signal quality during reception is added to an up-control channel signal from the mobile station and is transmitted to the exchange control station, and the exchange control station selects an up-control channel signal based on the received up-control channel signal and its signal quality.
Here, a general handover method will be explained.
FIG. 6 is a system configuration view for establishing a handover method in a conventional CDMA mobile communication technique.
FIG. 7 is a sequence view of a conventional handover method.
A conventional CDMA mobile communication system has an exchange control station 100, radio base stations 200A and 200B, and a mobile terminal device 300. The exchange control station 100 is a control station for conducting call processing control, radio/wire circuit control and member administration. The radio base stations 200A and 200B transmit and receive signals by using a radio wave diffused by a spreading code mentioned later for conducting communication with the mobile terminal device 300.
Next, a channel to be transmitted from the radio base stations 200A and 200B or the mobile terminal device 300 will be explained.
A pilot channel is a channel that is set by transmitting radio waves diffused by a spreading code from the respective radio base stations 200A and 200B so that an optimum area for the communication of the mobile device 300 can be selected. The spreading code of this channel is a product of codes BTS-k and a code PICH representing the pilot channel.
In the conventional CDMA mobile communication system, since the same carrier frequency is used for the communication between all of the radio base stations 200A and 200B and the mobile terminal device 300, it is necessary to allocate spreading codes different from each other for each of the radio base stations 200A and 200B so that the mobile terminal device 300 can discriminate the radio base stations 200A and 200B from each other. In FIG. 6, a spreading code BTS-A is allocated to the radio base station 200A, and a spreading code BTS-B is allocated to the radio base station 200B. In addition, the code PICH is the same as in all of radio areas.
The mobile terminal device 300 measures the electric field strength of pilot channels from the plurality of adjacent radio base stations 200A and 200B, and selects one radio base station most suitable for communication, for example, the radio base station 200A. The mobile terminal device 300 during conversation newly receives a pilot channel of a radio base station other than the selected radio base station 200A, and when the electric field strength of this newly received pilot channel exceeds a threshold value for staring up handover, it transmits a handover request message to the exchange control station 100.
An individual TCH is a channel for transmitting a voice signal and information or the like of an adjacent radio area between the mobile device 300 and the exchange control station 100. Since the plurality of mobile devices 300 are connected to one of the radio base stations 200A and 200B, a plurality of individual TCHs are prepared in the radio base stations 200A and 200B. A spreading code of the individual TCHs from these radio base stations 200A and 200B to the mobile terminal device 300 is a product of the codes BTS-k and codes DTCHn (Any value of 1, 2, 3, . . . is allocated to n. Same hereinafter.) that are different from each other for every mobile terminal device 300 within a radio area or a product of the codes BTS-k and codes DTCHm (Any value of 1, 2, 3, . . . is allocated to m. Same hereinafter.).
Also, the spreading codes of the individual TCH from the mobile terminal device 300 to the radio base stations 200A and 200B are codes MSj (Any value of 1, 2, 3, . . . is allocated to j. Same hereinafter.) that are different from each other for every mobile terminal device 300.
Next, how handover is conducted in the conventional CDMA mobile communication system configured above will be explained particularly.
In case that the mobile terminal device 300 relaying the radio base station 200A and communicating with the exchange control station 100 is moving towards the radio base station 200B, when it enters an area of a radio wave of the radio base station 200B, the electric field strength of a radio wave of a pilot channel originated from the radio base station 200B exceeds a threshold value for starting up handover.
If it exceeds this threshold value, the mobile terminal device 300 transmits a handover request message to the exchange control station 100 by means of the individual TCH (FIG. 7, step 1).
The exchange control station 100 that received the handover request message transmits a conversation channel start-up message to the radio base station 200B for starting up the individual TCH of the radio base station 200B (FIG. 7, step 2).
The radio base station 200B that received the conversation channel start-up message issues a conversation channel start-up affirmation message for notifying the exchange control station 100 of the spreading codes DTCHm to be newly used (FIG. 7, step 3).
For indicating to the mobile device 300 the spreading codes DTCHm to be used in the radio base station 200B that is a handover destination, the exchange control station 100 that received the conversation channel start-up affirmation message transmits a handover direction message to the mobile terminal device 300 using the individual TCH of the radio base station 200A that is a handover source (FIG. 7, step 4).
The mobile device 300 that received the handover request message issues a handover completion message by means of the individual TCH (FIG. 7, step 5, step 6). And, the exchange control station 100 receives the handover completion message, and thereby, the control of the handover ends.
However, due to an obstacle or the like between the radio base station 200A and the radio base station 200B, there is a case where the mobile terminal 300 cannot receive the handover direction message from the individual TCH of the radio base station 200A that is a handover source.
Below, circumstances will be explained in detail by using FIG. 8.
First, in case that the mobile terminal device 300 relaying the radio base station 200A and communicating with the exchange control station 100 is moving towards the radio base station 200B, when it comes from behind an obstacle 400, the electric field strength of a radio wave of a pilot channel originated from the radio base station 200B exceeds a threshold value for starting up handover. For example, a case where it comes from behind a building at a crossing and the radio base station 200B becomes to be seen corresponds to this.
If it exceeds this threshold value, the mobile terminal device 300 transmits a handover request message to the exchange control station 100 by means of an individual conversation channel (referred to as an individual TCH, hereinafter) (FIG. 8, step 1).
The exchange control station 100 that received the handover request message transmits a conversation channel start-up message to the radio base station 200B for starting up the individual TCH of the radio base station 200B (FIG. 8, step 2).
The radio base station 200B that received the conversation channel start-up message issues a conversation channel start-up affirmation message for notifying the exchange control station 100 of the spreading codes DTCHm that can be newly used (FIG. 8, step 3).
For indicating the spreading codes DTCHm to be used in the radio base station 200B that is a handover destination, the exchange control station 100 that received the conversation channel start-up affirmation message transmits a handover direction message to the mobile terminal device 300 using the individual TCH (the channel that is being used for the communication with the mobile terminal device 300) of the radio base station 200A that is a handover source (FIG. 8, step 4).
However, the mobile terminal device 300 that has come from behind the obstacle 400 cannot receive a signal from the radio base station 200A, since the electric field strength of a radio wave originated from the radio base station 200B is stronger than the electric field strength of a radio wave originated from the radio base station 200A.
Accordingly, the handover from the radio base station 200A to the radio base station 200B fails, and the connection is cut off.
In this manner, it is a phenomenon of the CDMA mobile communication technique in which the same carrier frequency is used that, when the electric field strength of a radio wave originated from the radio base station 200B abruptly becomes stronger than the electric field strength of a radio wave originated from the radio base station 200A, the mobile terminal device 300 cannot receive a signal being transmitted from the radio base station 200A, of which electric field strength of the radio wave is weaker. In addition, although a case where the obstacle 400 exists was explained above, the similar phenomenon also occurs in a case where a phasing phenomenon is caused in a radio wave from the radio base station 200A, and it abruptly becomes weaker than the electric filed strength of a radio wave originated from the radio base station 200B.